Densification of alloys

ABSTRACT

A process suitable for densifying metal-alloy powders such as oxide-strengthened metal alloy powder is disclosed which involves heat treating a shaped article from alloy powders in a reducing atmosphere to remove volatiles and oxygen from the matrix metal, providing a non-oxidizing atmosphere for the shaped article and working the alloy under heated conditions to reduce the thickness of the alloy to thereby affect a density increase. The process prevents undesired dispersed-metal oxide particle growth and clustering of the particles.

United States Patent Scheithauer, Jr.

[ Jan. 21, 1975 DENSIFICATION 0F ALLOYS [75] Inventor: William Scheithauer, Jr., Athens,

[73] Assignee: GTE Sylvania incorporated,

Stamford, Conn.

[22] Filed: Aug. 15, 1973 [21] Appl. No.: 388,392

[52] U.S. Cl l48/ll.5 F, 75/226 [51] Int. Cl B22! 3/00 [58] Field of Search 148/11.5 F; 75/206, 226

[56] References Cited UNITED STATES PATENTS 3,087,234 4/1963 Alexander et al. 148/1 1.5 F

Primary Examiner-W. Stallard Attorney, Agent, or Firm-Norman l. OMalley; Donald R. Castle; William H. McNeill [57] ABSTRACT A process suitable for densifying metal-alloy powders such as oxide-strengthened metal alloy powder is disclosed which involves heattreating a shaped article from alloy powders in a reducing atmosphere to remove volatiles and oxygen from the matrix metal, providing a non-oxidizing atmosphere for the shaped article and working the alloy under heated conditions to reduce the thickness of the alloy to thereby affect a density increase. The process prevents undesired dis-, persed-metal oxide particle growth and clustering of the particles.

5 Claims, No Drawings DENSIFICATION OF ALLOYS This application is a continuation of Ser. No. 185,901, filed Oct. 1, 1971,.and now abandoned and assigned to the assignee of the present invention.

CROSS REFERENCE TO RELATED APPLICATIONS In US. Pat. Application Ser. No. 185,902, filed concurrently herewith and assigned to the same assignee, there is disclosed a process for producing metal plates which may utilize the products produced by the subject matter of this invention as raw materials.

In US. Pat. Application Ser. No. 185,903, filed concurrently herewith and assigned to the same assignee, there is disclosed a process for controlling the particle size growth of metal oxides dispersed in a metal matrix during subsequent processing steps.

BACKGROUND OF THE INVENTION This invention relates to the densification of a powder metallurgyproduct. More particularly, it is related to a thermal-mechanicaltechnique for the hot densitication of preformed, porous shapes. It is more useful for, but not restricted to, the densification ofmaterials which cannot be densified by conventional powder metallurgy sintering techniques.

Densification by sintering is a useful technique for consolidating'manypowder metallurgy materials. However, some materials for one reason or another cannot be densified by sintering or if they can be the required temperature is so high as to destroy someof the desired characteristics of the material.

For example, the class of alloys known as oxide-dispersion-strengthened alloys are difficult to sinter. These alloys consist of either a pure metal or alloy powder, the oxides of which have a A F, at 27 C of from about 35 to 83 K cal/gram atom of oxygen which contains a finely divided, 500A) dispersion of refractory oxide particles (dispersoid) having a free energy of formation greater than 75 K cal/mole at 1,000 C. If compacted powders of these alloys are exposed to temperatures high enough to cause any significant densitication via sintering then the dispersion is unusually degraded, resulting from an increase in the particle size or severe agglomeration of the dispersoid particles.

Generally, two techniques have been employed for the consolidation of these hard-to-sinter materials. One such technique is referred to as hot-pressing. This technique is expensive and somewhat difficult to control. The size of the part to be densified is also limited. The technique that is most frequently used is hot-extrusion. In this technique the powders are either mechanically or isostatically compacted into a preform. The preform is then given a heat-treatment, generally in hydrogen. This preform is then sealed in an evacuated extrusion can. From this point on, the hot-consolidation procedure closely approximates conventional hot extrusion. The canned assembly is preheated, placed in the container of the press and then extruded to a round, rectangular, or non-regular cross-section, thus producing a cladded material. After extrusion, the can, or at this point cladding material, is removed and the resulting extruded material is generally conditioned by mechanical means prior to additional metalworking. This densification technique is expensive because of the canning and de-cladding operations that are required. Also, ex-

trusion is an inherently expensive process when not used for high-volume production.

It is believed that a process which produces a relatively dense form, suitable for subsequent metalworking operations, and at a reduced cost is an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to provide a process for the densification of alloy powders.

It is another object of this invention to provide a process suitable for dcnsifying certain metal powders that cannot be densified by normal sintering techniques.

It is a further object of this invention to provide a process capable of densifying certain metal powders sufficiently that their density approaches 100 percent of theoretical.

These objects as well as others are achieved by one aspect of this invention wherein a relatively porous shaped article of a material having a refractory metal oxide dispersed in a matrix metal having a theoretical densityless than about percent isheat treated in a reducing atmosphere to remove volatiles and reduce any matrix metal oxides present to metallic form without reducing the refractory metal oxide, providing a protective atmosphere of a non-oxidizing gas around the resulting heattreated article and thereafter metalworking the alloy articles, while hot, to a thickness of about 60 percent of its original to thereby increase the density of the article to at least 90 percent of theoretical.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims.

In the process of this invention powders of the hardto-sinter material are either isostatically or mechanically compacted into a round, rectangular or other regular shaped article. These compacted shapes are then heat-treated in hydrogen to a temperature sufficient to remove any volatile impurities and primarily oxygen. The heat-treated articles are then stored in a protective atmosphere until the time of hot consolidation. Preferably the articles are worked directly from the initial heat treatment. The articles are then pre-heated in a hydrogen atmosphere or other reducing atmosphere. The reducing atmosphere prevents the material, which at this point is about 50 to percent dense, from being oxidized. After heating for the desired time period, the form is quickly transferred to a metalworking device such as a drop forge, press forge, a rolling mill or other such devices known to those skilled in the art. The temperature is maintained at from about 800 to about 1,200 C. Through the combination of heat and pressure the form is quickly and easily increased to near theoretical density or at least a density sufficient to preclude any interconnecting porosity. Although at this point the article does not require a protective atmosphere, it is preferred to transfer it back to the hydrogen atmosphere furnace and reheated. Next, it is generally, but not necessarily, taken through another hot metalworking operation of either a similar or different type than the first. From this point on the material can be subjected to more or less conventional metalworking techniques to obtain the desired shapes, properties, and sizes.

EXAMPLE 1 Powders of Ni-ZThO are isostatically compacted at 40 ksi into a rectangular slab approximately 1 inch thick X 11 inches wide X inches long. At this point the density of the material is approximately 65 percent. The slab is next heat treated in hydrogen to a maximum temperature of about 1,050 C and cooled to room temperature in flowing hydrogen. The slab is stored in a sealed nitrogen container while awaiting the next operation. The slab is then preheated in a pure hydrogen atmosphere for approximately 60 minutes at about 1,050 C. It is then quickly transferred to the platens of a press forge and the load is rapidly applied. This generally effects an increase in density to at least 90 percent. The slab is next transferred back to the preheat furnace where it is reheated for about 10-30 minutes at about 1,000 C. The slab is transferred to a hot rolling mill where two 35 percent reductions are affected without a reheat between passes. At this point the slab is ready for conditioning and for subsequent metalworking operations.

EXAMPLE 2 Slabs of Ni-2Th0 approximately 1 inch X 4 inches X 8 inches are prepared in the same manner as in Example l. The slabs are preheated for about 60 minutes at about 1,050 C in hydrogen, and then quickly transferred to a rolling mill. Two successive passes are made such that each pass affects a thickness reduction of about 35 percent. The slab is then cooled in air. The density at this point is greater than 98 percent of theoretical and the slab is ready for conditioning and subsequent metalworking operations.

EXAMPLE 3 Powders of Cu-2Th0 are isostatically compacted 40 ksi into slabs 0.6 inch X 2.5 inches X 2.5 inches. The pressed densities are about 75 percent of theoretical. The slabs are heat treated in hydrogen by an anisothermal cycle to a maximum temperature of about 925 C.

After being held at about 925 C for about 1 hour. the slabs are quickly transferred to the platens of a drop forge where four rapid successive blows are given and then are air cooled. A thickness reduction of 28 percent is achieved which is sufficient to increase the density to 98 percent of theoretical.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

l. A process consisting essentially of a. heat treating a relatively porous shaped article having a refractory metal oxide dispersoid in a matrix of a matrix metal and a matrix metal oxide, said article having a density of less than about percent of theoretical, in a reducing atmosphere at a temperature and for a time sufficient to remove the volatiles and oxygen from the matrix metal and the matrix metal oxide without the reduction of said refractory metal oxide and without any significant densification of said shaped article,

b. providing a nonoxidizing atmosphere around the resulting heat treated article, and

0. working said heat treated article in an uncanned state at a temperature of from about 800 to about l,200 C to a thickness of from about 58 to about 72 percent of its original thickness without decreasing the width of said article to increase the density of the resulting slab to at least about percent of theoretical.

2. A process according to claim 1 wherein said refractory metal oxide is thorium oxide.

3. A process according to claim 2 wherein said matrix metal is nickel.

4. A process according to claim 2 wherein said matrix metal is copper.

5. A process according to claim 1 wherein the theoretical density after working is about 98 percent. 

2. A process according to claim 1 wherein said refractory metal oxide is thorium oxide.
 3. A process according to claim 2 wherein said matrix metal is nickel.
 4. A process according to claim 2 wherein said matrix metal is copper.
 5. A process according to claim 1 wherein the theoretical density after working is abOut 98 percent. 